1. Field of the Invention
The present invention generally relates to an electronic circuit and a communication system, and in particular, relates to an electronic circuit and a communication system having a differential amplifier circuit into which an input signal and a reference signal are fed, in which a signal relating to an average of the input signal acts as the reference Signal.
2. Description of the Related Art
Recently, a PON (Passive Optical Network) type FTTH (Fiber To The Home) is being used widely. It is demanded that the system receives an optical signal having variable amplitude transmitted from each home and amplifies the optical signal as an electrical signal.
For example, Japanese Patent Application Publication No. 2005-223638 discloses a receiver circuit used for a PON type optical communication. In the receiver circuit, a light-receiving element such as a photo diode (PD) receives an optical signal and outputs a current. A transimpedance amplifier (TIA) converts the current signal output by the PD into a voltage signal. A differential amplifier circuit differentially amplifies an output signal of the TIA and a reference signal relating to an average of the output signal of the TIA. The differential amplifier circuit outputs a high level when the output signal of the TIA is higher than the reference signal, and outputs a low level when the output signal of the TIA is lower than the reference signal. In the receiver circuit used for the PON type optical communication, amplitude of an optical signal is not constant. Therefore, the receiver circuit treats a signal relating to the average of the input signal as the reference signal, and outputs the high level or the low level by comparing the input signal with the reference signal.
A conventional receiver circuit receives a dummy signal in a preamble period that is a given time after an input signal is fed into, because the conventional receiver circuit needs a time in which a reference signal is stabilized to be an average of the input signal. However, it is demanded that the reference signal is stabilized to be a signal relating to the average of the input signal promptly in order to obtain more speedy optical communication. It is demanded that the reference signal is stabilized promptly. And it is demanded that the reference signal is stabilized even if high input signal or low input signal is ted into continuously. For example, it is demanded that the reference signal is controlled to be a constant value when an optical signal has variable amplitude, because a false operation may be occurred when a signal having variable amplitude is fed into before the reference signal gets to the constant value if the amplitude of the optical signal is variable.